1. Field of the Invention
This invention relates to amplitude adjustment devices such as amplitude compression devices and amplitude expansion devices, which are made with the MOS (i.e., Metal-Oxide Semiconductor) technology. In addition, this invention also relates to full-wave rectifiers, applicable to the amplitude adjustment devices, which are made with the MOS technology. Specifically, the devices are used for amplitude adjustment and rectification of audio inputs of digital audio systems.
This application is based on Patent Application No. Hei 10-180864 and Patent Application No. Hei 10-180865 both filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, amplitude compression/expansion devices are used for compressing and expanding signals of audio playback systems or audio reproduction systems. In the case of the automobiles, for example, drivers normally hear the noise due to the running of the automobiles when listening to the music which is played back with audio devices. So, if the drivers play back the music having a broad dynamic range such as the classic music, the drivers are hard to listen to piano sounds which are performed in pianissimo, for example. To improve such hardness in listening to the music in the automobiles, amplitude compression devices called “compressors” are used for the audio devices so that musical tone signals having small amplitudes are reproduced with a relatively large gain while musical tone signals having large amplitudes are reproduced with a relatively small gain.
There are provided three examples as the aforementioned amplitude compression devices, as follows:
FIG. 11 shows a circuit configuration for a first example of the amplitude compression device, which uses a voltage control amplifier. Herein, the voltage control amplifier 100 contains a multiplier, which is configured using bipolar transistors. The voltage control amplifier 100 adjusts an amplitude of an input signal Vin based on a control signal Cs. Thus, the voltage control amplifier 100 produces an output signal Vout in response to the input signal Vin. An amplitude detection circuit 110 is configured by a full-wave rectifier and a low-pass filter. The amplitude detection circuit 110 produces the control signal Cs in response to an amplitude of the output signal Vout. Normally, the bipolar transistors have base-emitter voltage characteristics, which show logarithmic characteristics. Using such characteristics, the voltage control amplifier 100 adjusts the amplitude of the input signal Vin.
FIG. 12 shows a circuit configuration for a second example of the amplitude compression device, which uses a gain switching amplifier. Herein, the gain switching amplifier 200 has a capability of switching over gains thereof based on control data Dc. In addition, an amplitude detection circuit 210 detects an amplitude of an output signal Vout. So, the amplitude detection circuit 210 produces the control data Dc in response to the detected amplitude. Incidentally, the gain switching amplifier 200 has a number of steps in changing the gains, which are called “gain steps”. Herein, the number of gain steps corresponds to a number of bits of the control data Dc.
FIG. 13 shows a circuit configuration for a third example of the amplitude compression device, which uses a digital signal processor (i.e., DSP). Herein, an input signal Vin is supplied to a DSP 310 via an analog-to-digital converter (or A/D converter). The DSP 310 detects an amplitude of the input signal Vin. Then, the DSP 310 performs non-linear amplification based on the detected amplitude, thus producing output data thereof. A digital-to-analog converter 320 (or D/A converter) converts the output data of the DSP 310 to an analog signal, which is output as an output signal Vout.
The aforementioned examples of the amplitude compression devices suffer from problems, as follows:
The first example of the amplitude compression device shown in FIG. 11 is designed such that the voltage control amplifier 100 is configured using the bipolar transistors, wherein amplitude compression is performed using the logarithmic characteristics of the bipolar transistors. So, it is impossible to manufacture the amplitude compression device in a form of an IC in accordance with the MOS process (or MOS technology). For this reason, the first example of the amplitude compression device suffers from a problem in which it has a limited range of application.
In the second example of the amplitude compression device, the gain switching amplifier 200 cannot change the gains thereof in a continuous manner. Therefore, the output signal should be made discontinuous in response to gain switching timings. Thus, the second example suffers from a problem in which it cannot produce the output signal which is “smooth”.
The third example of the amplitude compression device uses the DSP 310, which requires conversion from analog signals to digital signals and conversion from digital signals to analog signals. For this reason, the third example suffers from a problem in which it has a complicated circuit configuration.
By the way, full-wave rectifiers are known as devices that perform full-wave rectification on signal voltages to detect amplitude values of signals. FIG. 14 shows an example of a circuit configuration for the full-wave rectifier. The full-wave rectifier of FIG. 14 is mainly configured by a half-wave rectifier and an addition circuit of an inversion type. Herein, the half-wave rectifier is configured by resistors 110, 120, diodes D1, D2 and an operational amplifier OP1, while the addition circuit is configured by resistors 130, 140, 150 and an operational amplifier OP2. All of the resistors 110 to 140 have same resistance “R”, while the resistor 150 has resistance of “R/2”.
The half-wave rectifier is configured such that the diodes D1, D2 cancel voltage drops Vf in forward directions. Therefore, a half-wave rectified signal V′ increases in a positive direction from a ground level. For example, if an input signal Vin shown in FIG. 15A is applied to the half-wave rectifier, its half-wave rectified signal V′ is shown in FIG. 15B.
In the addition circuit of the inversion type which is configured by the resistors 130 to 150 having the aforementioned resistances respectively, it is possible to perform addition on the input signal Vin with a gain “−1”, while it is possible to perform addition on the half-wave rectified signal V′ with a gain “−2”. Therefore, an output signal Vout of the addition circuit is shown in FIG. 15C.
As described above, the full-wave rectifier is configured using two diodes and two operational amplifiers (OP1, OP2), wherein the half-wave rectified signal V′ is produced and is mixed with the input signal Vin so that the output signal Vout is created.
The aforementioned full-wave rectifier can be applied to an audio signal processing circuit in order to detect amplitudes of reproduced audio signals, wherein processing is performed in response to the amplitudes of the reproduced audio signals. Engineers wish to manufacture such audio signal processing circuit in a form of a LSI circuit in accordance with the CMOS process (where “CMOS” is an abbreviation for “Complementary Metal-Oxide Semiconductor”). However, it is impossible to form the diodes by the CMOS process. So, there is a disadvantage in that the diodes should be provided as external components which are attached to the LSI circuit.